Moessbauer spectroscopy of thulium hyperfine interactions and electronic relaxation with a 3He-4He dilution refrigerator

The Moessbauer effect of 169Tm was used to investigate electronic phenomena in Tm ions such as crystal field effects, electronic relaxation among crystal field states, non-integral valence states, magnetic order and Jahn-Teller distortions. The low temperatures necessary for these studies were obtained with a 3He-4He dilution refrigerator. The temperature dependence of the quadrupole interaction from 0.040 K to 1100 K was used in conjunction with optical data to extract crystal field parameters and wavefunctions for the zircon structure compounds TmAsO4, TmVO4, and TmPO4. The compounds TmAsO4 and TmVO4 undergo Jahn-Teller distortions at low temperatures, and possible crystal field parameters for the distorted phase of TmVO4 were also determined. The asymmetry of the quadrupole doublets observed for these compounds is indicative of electronic relaxation. Moessbauer spectra of thulium metal taken at temperatues from 0.065 K to 300 K showed similar asymmetric quadrupole doublets above the magnetic ordering temperature, T/sub N/ = 56 K, and complex hyperfine structure below T/sub N/. A previous multiple-site interpretation of such spectra, which failed to account for the asymmetric doublets above T/sub N/, was shown to also be inadequate at lower temperatures. The chalcogenide compounds TmS, TmTe, and TmSe also appear to show electronic relaxation effects in their Moessbauer spectra, but only the electronic ground state properties and the non-integral valence character of TmSe were of major concern for the present work